Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Conventional digital cameras are typically equipped with imaging devices (i.e., imaging elements or imaging pixel cells). An imaging device may be implemented as, for example, a charge-coupled device (CCD) or a complementary metal-on-silicon (CMOS) imaging device. A photodiode can directly convert photo-carriers generated by light incident on the surface of its semiconductor light-receiving element into a current. Such imaging elements are intended to capture images of objects illuminated by light levels that typically range from candle light (10 Lx) to direct sunlight (100,000 Lx). When taking images under lower illumination levels, such as street lighting (0.1 Lx), moonlight (0.01 Lx), or starlight (0.0001 Lx), some digital cameras may require the use of external lighting or flash units because of poor imaging sensitivity under such low light conditions.
To reduce the size and weight of camera modules for devices such as mobile phones and the like, the imaging devices used in such digital cameras have been increasingly miniaturized. The miniaturization reduces the area available for photoelectric conversion. Consequently, the efficiency of such imaging devices has been reduced. In addition, external lighting or flash units are often necessary to obtain images of acceptable quality even in daytime lighting conditions.
Photodiodes have difficulty adapting to low light levels because, among other reasons, the signal to noise (S/N) ratio of a photodiode is constant with respect to incident light intensity. The graphs in FIG. 1 illustrate the constant S/N ratio property of a photodiode's output. More specifically, the amount of charge in both an output signal S, denoted by a signal line 101, and noise N (e.g., thermal noise generated by signal charge fluctuation), denoted by a noise line 102, increase in linear proportion to incident light levels.
Phototransistors, as opposed to photodiodes, may be configured to amplify the output signal S to reduce the need for external lighting or the like and to advantageously reduce the influence of external noise generated by adjacent circuits, such as the switching noise generated by driving circuits. Like photodiodes however, the graphs of FIG. 2 further illustrate the constant S/N ratio property of an amplifying phototransistor. The amount of charge in both an output signal S (denoted by a signal line 201) and in the noise N (denoted by a noise line 202) are linear functions of incident light, as are the corresponding lines 101 and 102 of the photodiode. In fact, the signal line 201 and the noise line 202 may be obtained from the corresponding lines in FIG. 1 by simply moving the signal line 101 and the noise line 102 in FIG. 1 vertically to a position 204 that corresponds to the gain α.
The human eye can see objects with the help of only dim light, such as starlight or moonlight, without artificial lighting. The weak or dim light induces dark adaptation in the eye, in which the chemical reaction cycle of the photoreceptor molecules (rhodopsin) contained in rod cells amplifies optical signals. In dark adaptation noise does not become so high that objects cannot be recognized. In other words, the S/N ratio of optical signals in dim light situations is increased in the eye, because the optical signals are amplified in the eye while noise is not amplified. An imaging device that amplifies the S/N ratio of optical signals in such a way does not yet exist.